The invention relates to a method and to an apparatus for determining at least one patient or treatment or apparatus parameter during an extracorporeal blood treatment carried out in an extracorporeal blood treatment apparatus.
Specifically, though not exclusively, the invention can be usefully applied in dialysis treatments.
The parameters to be determined can be, for example, ionic dialysance or clearance, plasma conductivity, plasma sodium concentration, dialysis dose, depurated volume of patient body water, ionic mass balance, blood recirculation, fistula flow, etc.
During a dialysis treatment, it is desirable to monitor patient or treatment or apparatus parameters in order, for example, to provide a measure of treatment efficacy, preferably in a real time, non-invasive and automatic mode.
Several methods have been suggested for monitoring a dialysis therapy based, for example, on conductivity measurements of treatment fluids flowing in the extracorporeal blood treatment apparatus.
EP 0 291 421 discloses a method for determining the blood sodium level of a patient whose blood circulates through one compartment of an exchanger separated from a dialysis fluid by a semipermeable membrane; the conductivity of the dialysis fluid of equilibrium with the plasma is determined by: changing in a selected manner the conductivity of the dialysis fluid at inflow the exchanger, measuring the conductivity of the dialysis fluid at outflow from the exchanger, determining a lag time tL of the change in conductivity of dialysis fluid between inflow to and outflow from the exchanger, and determining an equilibrium conductivity value of the dialysis fluid for which the conductivity at outflow from the exchanger at an instant t is equal to the conductivity at inflow at an instant t-tL.
EP 0 330 892 describes a dialysis system comprising means for the conducting of dialysis liquid and blood respectively on either side of one or more membranes in a dialyzer, means for controlling one or more parameters of the dialysis liquid before the dialyzer, means for measuring at least one parameter of the dialysis liquid after the dialyzer, and means for a comparison of the parameter measured after the dialyzer with the set value of the corresponding parameter before the dialyzer; the values measured are used for the calculation of a blood parameter, which is altered as a function of the parameter measured, for the purpose of calculation and/or control of this blood parameter.
In the method disclosed in U.S. Pat. No. 6,156,002 for measurement of mass and energy transfer parameters (clearance and dialysance) in hemodialysis, a pre-determined amount of a substance whose dialysance is to be measured is added upstream of the dialyzer; the amount of substance not dialyzed in the dialyzer is measured downstream of the dialyzer by integrating the concentration over time; dialysance is calculated from the amount added upstream, the amount measured downstream and the dialysate flow; in case the substance is part of the dialysate the base concentration is subtracted during integration; the addition of the concentrate upstream of the dialyzer can be done manually or, alternatively by the mixing pump of the dialysis machine; instead of an increase of the concentration with a concentrate dilution with water can be used as well.
U.S. Pat. No. 6,187,199 shows a method for determining hemodialysis parameters during an extracorporeal blood treatment according to which the blood to be treated in an extracorporeal circulation flows through the blood chamber of a dialyzer divided by a semipermeable membrane into the blood chamber and a dialysate chamber, and dialysate in a dialysate path flows through the dialysate chamber of the dialyzer; the hemodialysis parameter can also be determined when no balanced state has yet been established; the method is based on the response of the dialyzer to a pulse function as inlet signal (pulse response) from the course over time of the physical or chemical characteristic quantity of the dialysate upstream and downstream of the dialyzer; the hemodynamic parameter is then determined from the pulse response of the dialyzer.
EP 0 920 877 provides a method for determining a parameter indicative of the effectiveness of an extracorporeal blood treatment carried out using a membrane exchanger, wherein the method includes the steps of flowing through the exchanger a treatment liquid having a concentration characteristic and of varying the value of the characteristic upstream of the exchanger for a time at the end of which the characteristic is returned to a nominal value; a plurality of values adopted by the characteristic downstream of the exchanger in response to the upstream variation is measured and stored in memory; the area of a downstream perturbation region is determined, which is bounded by a baseline and a curve representing the variation of the measured values with respect to time; then, the parameter indicative of the effectiveness of the treatment is calculated using the area beneath the upstream curve and an area beneath an upstream curve.
EP 0 658 352 makes available a method for determining a significant parameter of the progress of an extracorporeal blood treatment carried-out using a membrane exchanger, includes the steps of successively circulating three treatment fluids through the exchanger; each fluid has a characteristic linked to at least one of the significant parameters of the treatment; the value of the characteristic in the first fluid upstream of the exchanger is different from the value of the characteristic in the second fluid upstream of the exchanger, the latter being itself different from the value of the characteristic in the third fluid upstream of the exchanger; two values for each of the three treatment fluids are measured, respectively upstream and downstream of the exchanger, and at least one value of at least one significant parameter of the progress of the treatment is calculated from the measured values. This method is directed to determine the representative parameters of the progress of the treatment without, as a result, the patient having to be subjected for a long period to treatment conditions different from the prescribed conditions. This method permits a precise determination of the significant parameters of the progress of the treatment from measurements carried out at short time intervals. In this manner, the patient is exposed for only a very short time to a treatment fluid different from the prescribed treatment fluid (for example too high or too low in sodium) and the method can be carried out as often as necessary for an appropriate monitoring of the treatment session.
The prior art comprises a monitoring system, applied on a dialysis machine, which periodically measures the dialysis liquid conductivity at the dialyzer outlet, following a driven increment of the dialysis liquid conductivity at the dialyzer inlet. A processor receives said conductivity measurements and computes, by means of a mathematical model, several parameters relevant to the dialysis process, as ionic dialysance, plasma conductivity, kT/V, etc.
More in detail, for the measurement of ionic dialysance, a step increment is generated in the inlet conductivity by raising the conductivity by 1.0 mS/cm for 2 minutes and then reverting to the original conductivity. These 2 minutes have so far been sufficient to allow both the inlet conductivity and the outlet conductivity after the dialyzer to stabilize on the new level before going down again. It has thus been possible to find out the steady state level of the outlet conductivity that corresponds to each of the two inlet levels, which is used for the calculation of ionic dialysance.
With increasing demands on the quality of the dialysis fluid it has now become standard practice to offer machines that can clean the fresh dialysis fluid just before the dialyzer. The filter used for this (clean dialysate filter or ultrafilter) has to be fairly large to handle the flow of dialysis fluid, which can in some machines be up to 1000 ml/min. A large filter in the dialysis line creates an extra time period for stabilizing the outlet conductivity after the dialyzer, this extra delay in the response created by the clean dialysate filter being on top of the delay already created by the dialyzer, whereby 2 minutes may no longer be sufficient to allow reliable results. The large fluid volume in the clean dialysate filter acts as a mixing chamber that creates a very sluggish conductivity response after the filter. If the clean dialysate filter has, for example, a time constant of approximately 1 min (at 500 ml/min flow), it will correspond to a rise time (10-90%) of about 2 minutes, which is approximately equal to the duration of the afore mentioned conductivity setpoint step increment. Thus there is no chance to reach steady state conditions during the elevated conductivity period.
One solution to the problem with long time periods would be to increase the length of the conductivity step increment.
This would have several disadvantages.
One immediate disadvantage is that the mean conductivity in the inlet fluid will be changed, especially if, as normal, measurements are performed every 15 or 30 minutes. The treatment would no longer be performed with essentially constant conductivity, and the prescribing doctor might view this as a limitation.
Another disadvantage arises when the above-described monitoring system is combined with conductivity profiling, since profiling has to be prevented during and around the conductivity step increment very little time would be left for conductivity profiling.
Finally, the measurement accuracy would be decreased by the change in blood conductivity that would occur during such a long step, since changes in the blood conductivity affect the outlet conductivity.